From our previous experiments it may be concluded that environmental factors such as drugs when affecting the young embryo during its morphogenetic period cause gross malformations, but when affecting the fetus in later stages of development cause cellular abnormalities by interfering with proliferation, migration and differentiation. Despite the enormous repair in the CNS following subtle chemical insults, behavioral abnormalities such as learning and motor disturbances become apparent in postnatal life. These abnormalities are highly specific and can be related exactly to the time of the development the CNS was affected. Based on previous observations the aims of the proposed experiments are: a) To investigate the effects of prenatal chemical insults on higher level neural processes to relate experimentally produced cell deficits to learning and perceptual deficits and to relate abnormal behavioral patterns to those seen in congenitally brain damaged humans. b) To examine the response of the developing CNS to environmental and chemical insults with special attention to the repair mechanism and plasticity. c) To study the site and mechanism of action of different drugs and hormones on the developing CNS by applying dry and wet radioautographic and electronmicroscopic techniques. d) To explore the action of drugs able to interfere with cellular differentiation and to examine the affected neurons and their processes with silver straining and electronmicroscopical methods.